The present invention relates to a method of eliminating gel contained in polymer such as polyolefin by homogenizing and compounding the polymer, and also to a continuous kneader for realizing this method.
To eliminate gel contained in polymer such as polyolefin by homogenizing and compounding the polymer, the polymer is first charged from a hopper into a barrel provided with two rotors of various sectional shapes rotating in opposite directions to knead the polymer by rotation of the two rotors with a large shearing force, thereby plasticizing and melting the polymer as feeding.
FIGS. 6a and 6b are cross sections of rotor pairs having different sectional shapes in a continuous-type kneader used to knead the polymer. As shown in FIGS. 6a and 6b, the rotor pairs used in the continuous-type kneader are generally composed of triangular (three-wing) rotors and elliptic (two-wing) rotors. On the other hand, FIG. 7 in a cross section of a rotor pair in a batch-type kneader. As shown in FIG. 7, the rotor pair used in the batch-type kneader is generally composed of drop-shaped (one-wing) rotors.
FIG. 8 shows the operation of one of elliptic (two-wing) rotors in kneading polymer by rotating the rotors arranged in parallel. As shown in FIG. 8, a rotor 2 having two wings 3 is rotated in a direction of arrow P shown in FIG. 8 in a barrel 1 to apply a large shearing force to polymer as a stock to be kneaded in a wedge-shaped space 4 defined between the rotor 2 and the barrel 1, thereby plasticizing and melting the polymer. The polymer melted is repeatedly passed through a narrow clearance 5 defined between one of the wings 3 of the rotor 2 and the inner surface of the barrel 1, thereby homogenizing the polymer.
At this time, the tip of the wing 3 of the rotor 2 applies a large shearing force to the polymer, causing generation of a high pressure. Accordingly, the rotor 2 is subjected to a large force in a direction of arrow Q shown in FIG. 8 due to the high pressure, so that a bending stress due to this large force is applied to a shaft 6 of the rotor 2.
In a one-wing rotor, such a bending stress is always generated regardless of position of the wing. In a two-wing rotor, a bending stress generated by one of the two wings is canceled by a bending stress generated by the other wing. Accordingly, only when one of the two wings comes to a position at a communicating portion formed between adjacent barrel chambers, a bending stress is applied to a rotor shaft. In a three-wing rotor, one of the three wings is always present at the communicating portion, and receives reaction forces from the other two wings. However, these reaction forces are applied in such direction as to be canceled each other, so that a synthesized bending stress is small. Thus, the bending stress in a three-wing rotor is about 1/2 times that in a one-wing rotor or a two-wing rotor. For this reason, the use of a three-wing rotor is effective.
Further, since the tip of the wing applies a large shearing force to the polymer as mentioned above, a capability of dispersing aggregate of filler and gel in the polymer can be improved. Accordingly, it is general to use a two-wing rotor or a three-wing rotor in a conventional continuous kneader.
However, such a multi-wing rotor has a characteristic that a bending stress applied to the rotor is reduced and the amount of polymer passing through the clearances between the barrel and the tips of the wings is increased to thereby improve the dispersing performance; however, the work undergone by the polymer is increased to cause heat generation, resulting in a rise in temperature of the polymer. The rise in temperature of the polymer causes a decrease in viscosity of the polymer, so that the shearing action to the gel in the polymer cannot be effected and it is therefore difficult to eliminate the gel in the polymer.
Further, the multi-wing rotor has another characteristic that the volume occupied by the rotor in the barrel is increased to result in a decrease in volume of a kneading chamber defined between the barrel and the rotor, so that a time period of residence of the polymer in the kneading chamber is reduced. The reduction in such residence time of the polymer causes a problem that the polymer cannot be enough kneaded and it is therefore difficult to eliminate the gel in the polymer.
In this manner, the kneader used to eliminate the gel in the polymer is required to efficiently plasticize and melt the polymer just after supplied into the barrel and is also required to reduce the stress applied to the rotor. On the other hand, after plasticizing and melting the polymer, it is required to reduce the heat generation due to shearing by rotation of the rotor, thereby suppressing a temperature rise of the polymer, and is also required to increase the residence time of the polymer in the barrel. However, in the conventional continuous kneader, some rotor meets one of the above-mentioned required characteristics, but does not meet the other required characteristic, whereas the other rotor meets the other required characteristic, but does not meet the one required characteristic. Thus, a desired continuous kneader having rotors that meet both the required characteristics cannot be obtained in the related art.